1. Field of the Invention
This invention relates to a method of adding low-melting-point metals to molten steel. The invention particularly relates to such a method which, in the course of production, enables such low-melting-point metals as Pb and Bi to be added to molten steel uniformly and with good yield.
2. Description of the Prior Art
Examples of the conventional methods used for adding a low-melting-point metal to molten steel are illustrated in FIGS. 2 and 3. In the top-addition method shown in FIG. 2, stirring gas 3 is blown in through a porous plug 2 provided in the bottom of a vessel 1 containing molten steel 4. Particles of low-melting-point metal 5 added from the upper surface of the molten steel 4 are dissolved into the molten steel as it is stirred by the gas.
In the injection-addition method shown in FIG. 3, a lance 6 is immersed in the molten steel 4 and low-melting-point metal particles 5 are injected through the lance 6 together with a carrier gas 7 such as argon to be dissolved into the molten steel.
In addition to the foregoing methods, Japanese Unexamined Patent Publication No. 50(1975)-26722 discloses a method in which a powdered desulfurizing reagent is blown in together with a carrier gas through the lower portion of the side wall of a vessel containing the molten steel.
These prior art techniques suffer from the following disadvantages. In the top-addition method illustrated in FIG. 2, the addition-yield is low because the low-melting-point metal particles 5 evaporate and form smoke as soon as they reach the top surface of the molten steel. Moreover, as the smoke produced is detrimental to human health, it is necessary to employ large-scale dust collection equipment. Further, the added low-melting-point particles are entrained by the rising current of the molten steel 4 only for a very short time and are then quickly entrained by a down-current. Therefore, low-melting-point metals having a larger specific gravity than the molten steel, such as Pb and Bi, tend to settle on the bottom of the vessel without being adequately dissolved. As a result, the concentration of the low-melting-point metal becomes extremely high at the lower region.
In the injection-addition method shown in FIG. 3, the low-melting-point particles 5 injected into the molten metal 4 together with the carrier gas 7 cannot easily be entrained by the upward current and, similarly to the case of the top-addition method, tend to settle to the bottom. Also, as the low-melting-point metal has a low melting point, the surface of the particles softens, so that clogging is apt to occur at bends in the piping and in the vicinity of the outlet of the immersed lance 6. Besides it costs a lot to equip and operate the immersed lance 6 etc.
As regards the invention disclosed in Japanese Unexamined Patent Publication No. 50(1975)-26722, it relates to blowing-in of a desulfurizing reagent and makes no mention of blowing-in of low-melting-point metal.